Synucleinopathies are a diverse group of neurodegenerative disorders that share a common pathologic characteristic: in neuropathologic examinations characteristic lesions can be detected containing abnormal aggregates of alpha-synuclein (alpha-syn, a-syn) protein in selected populations of neurons and glia cells.
Alpha-syn (initially identified as PARK1 and PARK4) is a 140 amino acid protein widely expressed in the neocortex, hippocampus, dentate gyrus, olfactory bulb, striatum, thalamus and cerebellum. Alpha-Syn is also highly expressed in hematopoietic cells including B—, T—, and NK cells as well as monocytes and platelets. The exact role in these cells is not known but it has been implicated in the differentiation of megakaryocytes (platelet precursors).
The most common synucleinopathies include but are not limited to Lewy body disorders (LBDs) like Parkinson's disease (PD), Parkinson's disease with dementia (PDD) and dementia with Lewy bodies (DLB), as well as Multiple System Atrophy (MSA) or Neurodegeneration with Brain Iron Accumulation type I (NBIA Type I). The current treatment options for these diseases include symptomatic medications such as L-dopa, anticholinergic drugs as well as inhibitors of monoamine oxidase. However, all treatment opportunities currently present only lead to symptomatic alleviation but do not induce a long lasting, disease modifying effect in patients.
Lewy body disorders (LBD) are progressive neurodegenerative disorders characterized by tremor, rigidity, bradykinesia and by loss of dopaminergic neurons in the brain. In the case of DLB and PDD signs also include cognitive impairment. Up to 2% of the population above 60 years of age in western countries develop the typical signs of PD/LBD. Currently only symptomatic treatment is available. Unfortunately, these therapies only provide temporary relief from early symptoms and do not halt disease progression. The pathogenesis of PD/LBD is still incompletely understood, but it appears that genetic susceptibility and environmental factors are involved in the development of the disease. Despite all genetic advances, PD/LBD is primarily a sporadic disorder with no known cause (also called idiopathic PD/LBD).
Patients suffering from this disease develop characteristic ubiquitinated intracellular inclusions called Lewy bodies (LBs) in the cortical and subcortical areas of the brain. Especially regions with high content of dopaminergic neurons or neuronal projections show this typical pathologic feature. Recently, several studies could show that the synaptic protein alpha-syn plays a central role in LBD pathogenesis. In LBD, alpha-syn accumulates in LBs throughout affected brain areas. Additionally, it could be demonstrated that single point mutations as well as duplications or multiplications in the alpha-syn gene are associated with rare familial forms of parkinsonism. Importantly, based on results from overexpression studies in transgenic (tg) mice as well as in Drosophila melanogaster its key role in the pathogenesis of PD/LBD is underscored as these animal models mimic several characteristics of PD.
Another very important synucleinopathy is Multiple System Atrophy (MSA). MSA is a sporadic neurodegenerative disorder that is characterised by symptoms of L-DOPA-resistant parkinsonism, cerebellar ataxia, and dysautonomia. Patients suffer from multi-system neuronal loss affecting various brain areas including striatum, substantia nigra, cerebellum, pons, as well as the inferior olives and the spinal cord. MSA is characterized by al-pha syn-positive glial cytoplasmic (GCI) and rare neuronal inclusions throughout the central nervous system. These inclusions are associated with striatonigral degeneration, olivopontocerebellar atrophy, and involvement of autonomic nuclei in medulla and spinal cord. The importance of GCIs for the pathogenesis of MSA is generally acknowledged and underscored by recent analysis of transgenic mouse models analysing the effect of alpha-syn overexpression in oligodendroglia. In tg mice overexpressing human alpha-syn both GCI-like aggregates and biochemical markers of MSA were observed.
Although the exact mechanisms by which accumulation of al-pha-syn leads to the typical features of neurodegeneration in synucleopathies are not fully understood, recent studies imply that abnormal formation and accumulation of alpha-syn is involved in the degenerative processes underlying synucleinopathy. Recently, different forms of alpha-syn have been identified in LBs. Beside the full length form of the protein, different forms of modified alpha-syn have been identified including phosphorylated, nitrated, and mono-, di-, or tri-ubiquitinated alpha-syn. In addition, C-terminally truncated forms of the protein, like alpha-syn 1-119, alpha-syn 1-122 and alpha-syn 1-123, have been detected in brain tissue from both transgenic mice and PD cases. It is currently believed that up to 15% of the alpha-syn detected in LBs and lewy neurites is truncated. Previous in vitro studies using truncated alpha-syn could demonstrate that alpha-syn lacking the C-terminal 20-30 amino acids was showing an increased tendency to aggregate and to form filaments found in Lewy-neurites and LBs. These truncated versions could thus act in a similar way as truncated and modified forms of amyloid beta (Aβ) in Alzheimer's disease (AD). These truncated and modified forms of Aβ are thought to act as seed molecules for plaque deposition and show a higher aggregation propensity as well as high neurotoxicity and synaptotoxicity in vivo and in vitro.
Thus full length alpha-syn as well as truncated and/or modified forms of alpha-syn, which are showing potential seeding effects, are then believed to accumulate leading to oligomer-formation. Based on recent studies it is believed that such oligomer-formation for example in the synaptic terminals and axons plays an important role for PD/LBD development and could thus be enhanced by the presence of truncated forms of alpha-syn. Hence, reduction of alpha-syn deposition and oligomerisation should be beneficial in the treatment of synucleopathies, especially of idiopathic LBD/PD and MSA and could present the first strategy for treatment of these neurodegenerative diseases in addition to the mere alleviation of symptoms resulting from current treatment strategies like L-DOPA application.
In Iwatsubo T. (Neuropathology 27 (5) (2007): 474-478) the correlation of alpha-synuclein depositions as well as its phosphorylation with a pathogenesis of alpha-synucleopathies is examined. The author of this publication found that serine 129 of alpha-synuclein deposited in synucleopathy lesions is extensively phosphorylated. US 2007/213253 relates to mutant human alpha-synuclein as well as peptides derived therefrom which may be used for inhibiting the aggregation of the wild-type human alpha-synuclein. In the WO 2004/041067 means and methods for preventing or treating diseases associated with alpha-synuclein aggregation are disclosed which comprise the use of alpha-synuclein fragments. In the US 2003/166558 peptides are described which can be used to induce immune response to protein deposits. US 2005/198694 relates to alpha-synuclein fragments comprising at least 100 amino acids and having a C-terminal deletion of 1 to 23 amino acids.
Liang et al. (J. Neurochem. 99 (2006): 470-482) studied the regulation of alpha-synuclein in rats. They observed that in alcohol preferring rats the expression rate of alpha-synuclein is increased compared to alcohol-non preferring rats.
In Hamilton B A (Genomics 83 (2004): 739-742) the distribution of alpha-synuclein 53Thr and 53Ala in primates is examined.
In US 2005/0037013 immunogenic alpha-synuclein fragments are disclosed which are able to induce an immune response against a specific epitope within residues 70-140 of alpha-synuclein.
WO 2006/045037 relates to C-terminal truncated alpha-synuclein molecules which can be used to screen for agents which have a pharmacological activity useful for treating a Lewy Body Disease.
Although experimental therapies utilizing neurotrophic factors and grafting of dopaminergic cells have yielded promising results, alternative approaches designed to reduce the neuronal accumulation of alpha-syn are required. There is compelling evidence accumulating that alpha-syn aggregates might be targeted by immunotherapy. Indeed, recently a potential for the treatment of synucleopathies has been shown. Tg mice overexpressing human alpha-syn were vaccinated with human alpha-syn protein. In mice that produced high relative affinity antibodies upon vaccination, there was decreased accumulation of aggregated alpha-syn in neuronal cell bodies and synapses which was associated with reduced neurodegeneration. Furthermore, antibodies produced by immunized animals also detected abnormal aggregated forms of alpha-syn associated with the neuronal membrane and promoted the degradation of these aggregates, probably via lysosomal pathways. Similar effects were observed using passive immunotherapy with an exogenously applied alpha-syn-specific antibody. These results suggest that vaccination is effective in reducing neuronal accumulation of alpha-syn aggregates and that further development of this approach might elicit beneficial effects in the treatment of LBD and synucleinopathies.